Insulator having semi-conductive layers to increase the capacitance thereof



c. H. w. CLARK 3,243,505 INSULATOR HAVING SEMI-CONDUCTIVE LAYERS TO INCREASE March 29, 1966 THE GAPACITANCE THEREOF Filed Feb. 17, 1964 FIG. l.

FIG. 2'.

United States Patent C) 7 l 3,243,505 INSULATOR HAVING SEMI-CONDUCTIVE LAYERS'- T0 INCREASE THE CAPACI- TANCE THEREOF' y C'olin` Henry William Clark, Stourport-on-Severn, England, a'ssignorto'Steatite and Porcelain Products Limf ited, London, England, a corporation of Great Britain L FiledFeb. 17, 1964, Ser."No. 345,212. Claims priority, applicationGreatfritain, Feb. 25, 1963,

6 Claims. (Cl. 174-140) interference.n 'v

l'fIn such insulator. assemblies the `electrically conducting bodies and their'f'associated semi-conducting fringes act as condenseriplates'; The capacitance of such a condenser is VsmallI and'thaty of the" assembly not appreciably different from the capacitance of a simil-ar .assembly withoutlsemiconducting fringes.` When'such'insulator assemblies are connected inseries'in the form of a practical high voltage l'rsula'torthev istrib'ution of the total applied voltage bet 1r them isvgoverned by the capacitance `of'each assembly;Y thestrayfcapacitance to nearby objects and by moistureand other vcontamination on the exposed surface of the insulatingfbody. The eifectsl ofv stray capacitance and ofthe leakage *currents duetoI surface contamination are vnnfav'ourable",tosatisfactory performance of the insulator.

The present invention provides an insulator assembly comprisingonefor av stackcfr'nore than one insulating body, each havin-g electrically conducting 'material atta'clied toopposite sides, each insulating body having, at or near its, surface,l layersl -of semi-'conductingv material underlying, electrically connected to and extending beyond thesaid'electrically conducting material in which the area A of the semi-conducting layers is sufliciently large to increase the electrostatic capacitancefof the assembly to at least 2.5 times the capacitance of a similar assembly without semi-conducting layers. Preferablythe capacitance is increased by at least live times. v

Within the term felectrically conducting material we wish to include metal-lic components such Ias support littings and wires carrying current and alsol cement which is used for joining and which is electrically conducting. For example, Portland cement is frequently used and if a measurement is made of the electrical resistivity o-f a typical mass of set Portland cement it is found that the volume resistivity is of the order of 104 ohm cms. This compares with the volume 'resistivity of an insulating material such as porcelain which is at least 1014 ohm cms. For semiconducting layers such as semi-conducting glazes used on porcelain insulators a different measure of conductivity is used, namely the surface resistivity or resistance between opposite faces of a square of the surface. Normal porcelain has a surface resistivity of the order of 1011 ohms, due to the adsorbed moisture -lm; the surface resistivity of a semi-conducting glaze or semi-conductinglayer may be in the range of 103 to 108 ohms. t

One method of manufacturing high voltage insulator assemblies is to form a number of cup-like insulating bodies known in the trade as shells. A shell may be cemented to metal fittings, for example a metal cap 0n the outside of the shell and a metal pin within the shell, to for-m a single-shell insulator assembly. Two or more shells may be cemented together and to metal fittings to form a multi-shell insulator assembly. Such insulator assemblies, whether of the single-shell or the multi-shell y construction, may also be joined together, for example by bolts engaging the metal fittings, to form multi-'assembly insulators suitable for higher service voltages. The invention is especially applicable to, assemblies of such shelltype insulating bodies. i n f Shell-type insulating bodies are usually of roughly sim.- ilar proportions because of factors governed for'example by production methods and mechanical properties, so although the size and shape may vary, it is usually true t0 say that a tive-fold increase in capacity is obtained when the semi-conducting layers extend over roughly half the surface of the insulating body. The actual values of the capacitance in each case are of course proportional to the areas of the conductors and associated semi-conducting layers and inversely proportional to the` thickness of the insulating body. For a flat insulatingbody of given thickness and having conducting of diameter 4 inches, for example, the capacitance would be increased `by five times by a circular semi-conducting layer of diameter approximately 9finches'. The vcapacitance of the shell could be increased by reduce ing the thickness, but 'the leads to difliculties in manufacture and the thin section is, of course, weaker and less likely to withstand the electrical and mechanical stresses imposed on it in service.

Hitherto an increase'in the area of the electricallysconducting material to achieve the desired increase-in cav pacitance has been avoided because the lengthof creepage pathuover the remaining part 0f the'ins'ulator surface is thereby reduced. ASuch a reduction of creep'age distance, if due to an extension. of the fully conducting electrodes such as the metal littings, would certainly be a serious disadvantage and would increase the tendencylfor vdischarges to occur by flashover across the surface, orfor currents to leak across the surface.

We have found that this tendency is avoided byjusing semiconducting layers having a 4suitably high-surface resistivity. Preferably the surface resistiviity is more than about 2X l05 ohms per square. If the surface resistivity is too high however, the sem-i-conducting layer becomes ineffective and the resistivity should not therefore exceed about 20 106 ohms per square.

In practice satisfactory resistivity can be achieved when the semi-conducting layercomprises lblue titania.

In insulator assemblies provided by the present invention the capacitance of each insulating body isI sutiicient to give a worth-while stabilisin-g current in polluted com ditions without seriously reducing the effective creepage distance over the surface of the insulating body.

Insulator assemblies -according to the present invention are especially useful when sever-al assemblies are joined Patented Mar. 29, 1966y together -to form a long multi-assembly insulator string oripost. electrical capacitance in order to obtain the full benefit of the invention. In known insulators having a restricted semi-conducting fringe in order to reduce corona discharge the limited effect sought is obtained even if only a few assemblies at the high voltage end of the string or post are so treated.

The semi-conducting layer may be covered with a protective layer of porcelain glaze or it may form part of a glaze. It should of course be in a permanent form able to resist weather and electrical discharges.

It is believed, without prejudice ,to the invention, that the improved performance of the insulators provided by the invention is due to the large capacitance which conltrois the voltage gradient across the insulators even when their surfaces are wet and dirty. Whereas in ordinary insulators the voltage gradient is controlled by local variations in surface conditions and stray capacitance which allow local high voltage stresses to be set up, in our insulators the large capacitance renders local effects negligible or of less importance.

Preferred embodiments of the invention will now be particularly described by way of example onlyl and lwith reference to the accompanying drawing in which:

FIGURE. lis a part sectional elevation of an insulator assembly comprising a single insulatingbody and FIGURE 2 is .a similar view of an insulator assembly comprising two insulating bodies cementedtogether.

Referring to the drawing, a porcelain shell-type insulatingbody 1A has surface layers 2, 3 comprisingsemiconducting blue titania on its top and bottom surfaces, the layers extending radially outwards to positions 7 and together covering about" half the total surface of the insulating body. An electrically conducting metallic cap 4 is cemented to a central part of the top layer 2 by electrically vconducting cement 5 and makes electricall contact with the layer at the top of the body 1. A metallic pin is similarly cemented to `the bott-om layer 3. The insulator assembly so formed is suitable for forming 'a part of a string of similar assemblies having an earthed structure at one end and a conductor carrying a very high voltage at the other. In such strings Ithe pin of one .assembly ts into a socket in the cap of the adjacent assembly. Each assembly may have two or more shells cemented together in a stack. The approximate dimensions of the insulator assembly are:

Diameter of insulator body 1 inches 14 Diameter of semi-conducting layers 2, 3 do 10 Internal diameter of cap 4 do y 41/2 Diameter of pin 6 and cement do 3 Capacitance picofarads 150 Capacitance of similar assembly without semi-conducting layers v do 30 The surface resistivity of the semi-conducting layer was approximately 3 l05 ohms and this was found to give no'ser-ious reduction of the flashover voltage of the clean insulator. To demonstrate this, tests were carried out on a string of ve cap and pin suspension insulator assemblies (similar to that shown in FIGURE 1). Flashover tests were made using a power frequency voltage with a frequency of 50 cycles per second and also using a 1.2/ 50 impulse voltage wave (an impulse voltage which rises to full value in v1.2 microseconds and decays to half value in 50 microseconds). In Table I the results are compared with those obtained on a similar string of insulator units with no conducting layers. On longer strings of more than ve insulator units the ashover voltages were actual'- 'ly higher for the insulator units of the present invention because'the voltage grading on the untreated ones was non-uniform even in' clean conditions owing to stratl capacitance effects. e

In this case every assembly should have a Alarge able I With semicon- Without semlducting conducting layer, kv. layer, kv. 5

Power frequency Ilashover voltagedry 354 360' Power frequency ashovcr voltagew 233 245 Impulse flashovor voltage 565 560 FIGURE 2 represents a two shell insulator assembly comprising two insulating bodies 1 generally similar to those shown in FIGURE l, stacked ,together and joined with conducting cement 5 and having electrically lconducting metal fittings 4, 6 cemented tothe top andbottom respectively. The anges of several assembliesvcanbo bolted together end to end to form a 'postinsl-llator Using the insulator assemblies provided by the present invention it is found that serious discharges kdue to pollution of the insulator surface are less likelyv to occur when very high voltages are being transmitted,

I claim: v

1. In an assembled electrical yinsulator comprising at least one insulator body,I electrically conductive vmaterial at and connected to each of opposite sides of eachzsaid body, and a layer of semi-conductive,*material -iny-the surfaceregion of each of said sides-and electrically man.` nected to and extending marginally beyond `,said electrical: ly conductive material, the improvement whereinthe-arca of each said layer of semi-conductive material-is such that the electrostatic capacitance ofthe insulator `is at least 2.5 times the capacitance which-the insulator would have without-said semi-conductive layers. i

2. The insulatorof claim 1, said capacitancel of the r insulator being at least Stimes .the capacitance which the insulator would have without sai-d semi-.conductive layers;

3. The insulator of claim 1, said semi-conductive-laycrs having a surface resistivity of between.2 10` .ohm and 2O 106 ohm. v f A V- 4. An assembled electricalvv insulator compriSi-ngat least one insulator shell, electrically conductive material at and connected to each of the opposite sides ofeach said shell and a layer of semi-conductive material in the surface region of each of said sides and electrically connected to r and extending marginally bey-ond said electricallysconcluctive material, the area of each said layer of semi-coriduc tive material being substantially an undivided `onefhalf of the area 4of the respective said side whereby `the electro.- static capacitance of the insulator is substantiallylSy times said' semi-conductive layers. y 5. The insulatorof claim 4 wherein saidserniaconduc? tive material has a surfaceresistivity of between 2x10? 55 `6. The insulator of claim 4 tive material is blue titania.

References Cited by the Examiner l UNITED STATES PATENTS LARAMIE E, ASKIN, Primary Examiner.

ROBERT K. SCHAEFER, Examiner. v

the capacitance which the insulator would have without t wherein said semi-coudll- 

4. AN ASSEMBLED ELECTRICAL INSULATOR COMPRISING AT LEAST ONE INSULATOR SHELL, ELECTRICALLY CONDUCTIVE MATERIAL AT AND CONNECTED TO EACH OF THE OPPOSITE SIDES OF EACH SAID SHELL AND A LAYER OF SEMI-CONDUCTIVE MATERIAL IN THE SURFACE REGION OF EACH OF SAID SIDES AND ELECTRICALLY CONNECTED TO AND EXTENDING MARGINALLY BEYOND SAID ELECTRICALLY CONDUCTIVE MATERIAL, THE AREA OF EACH SAID LAYER OF SEMI-CONDUCTIVE MATERIAL BEING SUBSTANTIALLY AN UNDIVIDED ONE-HALF OF THE AREA OF THE RESPECTIVE SAID SIDE WHEREBY THE ELECTROSTATIC CAPACITANCE OF THE INSULATOR IS SUBSTANTIALLY 5 TIMES THE CAPACITANCE WHICH THE INSULATOR WOULD HAVE WITHOUT SAID SEMI-CONDUCTIVE LAYERS. 